1. Field of the Invention
The present invention relates to an optical head for an optical disk such as a video disk, an audio disk, an optical memory disk, and the like, and more particularly to an objective lens used in the optical head.
2. Description of the Prior Art
The objective lens used in an optical head requires correction of aberration to the diffraction limit. Additionally, the objective lens is desired to be small in size and light in weight so as to be capable of moving quickly during the tracking or focusing servo-operation. Hence, a single aspherical lens has been proposed as disclosed in U.S. Pat. Nos. 4,027,952, 4,449,792, 4,571,034, and 4,655,556, and Japanese Patent Applications Laid-Open Nos. 57-76512 and 62-59912.
The conventional objective lens for optical head has the following disadvantage. Since the laser diode oscillates at approximately a single wavelength, the objective lens is designed to correct for aberration in respect to the one wavelength. Generally, the corrected aberration of the lens to the single wavelength changes less even if the oscillation wavelength of the laser diode varys in the order of about 10 nm. This is because the focal point of the lens is kept compensated by the servo-operation of auto-focusing.
However, in the erasable optical disk system, the power of the laser diode must be changed. For example, the power of 3 mW is required for reading data from the optical disk while the power of 40 to 60 mW is needed for erasing the recorded data. When the laser power is changed, the wavelength of the laser diode output will often vary instantly due to a mode hop. The variation ranges 5 nm to 10 nm in general. Since one of the reading, recording and erasing modes is switched to another quickly, the wavelength of the laser diode output varies in a moment. The wavelength change caused by the power variation of the laser diode often exceeds the controllable range of the optical head by the auto-focusing servo system, so that the displacement of the focal point due to chromatic aberration of the lens cannot be compensated. This causes a failure of reading data just after a change in the wavelength.
For solving such a problem, lenses corrected for chromatic aberration have been proposed in Japanese Patent Application Laid-Open Nos. 60-232519 and 62-217214. These lenses each consist of 4 to 5 elements, and thus will be high in cost and require a lens barrel which is finished with high accuracy. Additionally, the lens itself becomes heavy in weight and energy-saving in the entire arrangement will be difficult.